Ammonia is a valuable commodity. It can be used in fertilization of crops, as a refrigerant, in the manufacturing of other materials, in cleaning solutions, and in other applications. Ammonia is also difficult to form. The elementary steps involved in ammonia synthesis were elucidated by Nobel-prize winner Gerhard Ertl and coworkers in Primary steps in catalytic synthesis of ammonia, Journal of Vacuum Science & Technology A, 1(2): p. 1247-1253 (1983). Ammonia synthesis requires a sufficient amount of energy to break the triple nitrogen-nitrogen bond to form atomic nitrogen and the hydrogen-hydrogen bond to form atomic hydrogen. The atomic nitrogen and atomic hydrogen must then react to form ammonia. Furthermore, the reaction equilibrium does not favor the formation of ammonia resulting in fewer ammonia molecules compared to the input of atomic hydrogen and atomic nitrogen. Thus, the reaction must occur at high temperature and pressure. The high temperature and pressure requires significant energy, accounting for between about 1 and 2% of the energy supply worldwide. Additionally, the conversion rate using conventional techniques is poor, typically between 10-15% per pass, which requires a majority of the gases to be recycled and additional power, pressure, and temperature sources to be consumed.
Commercial ammonia synthesis relies on the Haber-Bosch process, which uses a potassium-promoted iron catalyst in an energy intensive process that has remained largely unchanged for a hundred years. The equilibrium constant of this exothermic reaction becomes unfavorable above 200° C., but the catalyst requires temperatures of ˜400° C. to have sufficient activity. To overcome these conflicting requirements, the process is conducted at extremely high pressure (greater than 200 atm) in multiple steps with interstage cooling to achieve sufficient conversion. Operating at such a high pressure and temperature increases the cost associated with the process. A long standing scientific challenge has been to achieve NH3 synthesis at ambient pressure.
Electrochemical routes have been pursued to achieve NH3 synthesis at ambient pressure. In electrochemical reactions, protons are driven by an applied voltage across an electrolyte where they recombine with N2 and electrons at the cathode to produce ammonia. These electrochemical routes for NH3 synthesis were reviewed by Giddey et al., Review of electrochemical ammonia production technologies and materials, International Journal of Hydrogen Energy, 38(34): p. 14576-14594 (2013)) who found them promising but current production rates are extremely low, with a maximum flux of 5·10−4 mol/m2s due to low conductivity (flux) of mixed salt/oxide electrolytes to hydrogen.